Generally, for forming an integrated circuit such as an IC or a logic device, there are repeatedly performed a process for forming a desired thin film on a surface of a semiconductor wafer, a glass substrate, an LCD substrate or the like; and a process for etching it in a desired pattern.
For example, in a film forming process, a specified processing gas (source gas) reacts in a processing vessel to form a silicon thin film, a thin film of a silicon oxide or nitride, or a metal thin film, a thin film of a metal oxide or metal nitride, or the like. As a result of the film forming process, residual reaction by-products are produced to be discharged with an exhaust gas. In addition, an unreacted processing gas is discharged.
If the reaction by-products or the unreacted processing gas are discharged into the air as they are, they may cause environmental pollution and the like. For preventing this, conventionally, a trap apparatus has been installed in an exhaust gas system extending from a processing vessel. In this way, the reaction by-products, the unreacted processing gas and the like, which are contained in the exhaust gas, have been collected and removed.
Various configurations of the trap apparatus have been proposed based on characteristics of the reaction by-products and the like that must be collected and removed. For example, in case of removing reaction by-products, which are condensed (liquefied) and/or coagulated (solidified) at room temperature, the trap apparatus can be configured such that multiple fins are provided in a housing having an inlet opening and an outlet opening of the exhaust gas. The fins are sequentially arranged along a flow direction of the exhaust gas, and the reaction by-products in the exhaust gas are collected by surfaces of the fins by being attached thereon when the exhaust gas passes between the fins. Further, the fins have been cooled by a cooling fluid to increase collection efficiency.
Here, as an example, a case where a Ti metal film is formed by using as a source gas a refractory metal halogen compound, i.e., TiCl4 (titanium tetrachloride), will now be explained. H2 gas as well as TiCl4 is used as the source gas, and these gases are activated by a plasma in the presence of Ar gas to thereby be reduced by hydrogen. As a result, the Ti film is deposited on a surface of the semiconductor wafer. During the process, TiClX (X<4) is produced as a reaction by-product and there also exists an unreacted TiCl4 gas, which are discharged as the exhaust gas. Since these TiClX and TiCl4 are impurity gases causing air pollution and the like, they are trapped by such a trap apparatus.
Here, the impurity gases such as the above-described TiCl4 as the unreacted gas, TiClX as the reaction by-product and the like, have relatively high vapor pressures, so that it is very difficult to completely trap and remove them in the trap apparatus, even when the trap apparatus is cooled as mentioned above. Thus, satisfactory recovery rate could not be realized. For completely removing the impurity gases passing through the trap apparatus and making them harmless, there has been provided a waste gas scrubber at a downstream side of the trap apparatus, which gives rise to soaring running cost of the waste gas scrubber and shortening of life span thereof. Such problems are common to the film forming apparatus, which employs a refractory metal compound gas such as TiCl4, WF6, (Ta(OE)5)2 or the like.
As for another film forming method using TiCl4, there has been known a method for forming a TiN film. Namely, as an example, a case where a TiN film is formed by using as a source gas a refractory metal halogen compound, i.e., TiCl4 (titanium tetrachloride), will now be explained. NH3 gas as well as TiCl4 is used as the source gas, and both gases react with each other to deposit the TiN film on a surface of the semiconductor wafer. At this time, NH4Cl or TiCl4(NH3)n (n is a positive integer) is produced as the reaction by-product, and an unreacted TiCl4 gas is also present. These gas components are discharged as the exhaust gas to be trapped by the aforementioned trap apparatus.
Further, in order to more completely remove the impurity gas, e.g., a chloride gas, contained in the exhaust gas, there has been proposed a method for efficiently removing the impurity gas by mixing the exhaust gas in a gas exhaust system with a reactive gas, e.g., an ammonium gas, which reacts with the impurity gas, to convert the impurity gas into, e.g., an ammonium chloride, which is likely to be condensed; and by cooling and condensing the ammonium chloride in the trap apparatus to be trapped therein (Japanese Patent Laid-open Application No. 2001-214272).
Still further, in Japanese Patent Laid-open Application No. S62-4405, there has been disclosed a technology wherein circular trap plates having small holes are placed in multi-levels inside a wax trap apparatus such that an exhaust gas passing through the small holes is cooled by a self-cooling while being adiabatically expanded, to thereby liquefy a wax to be collected, when liquefying the wax in the exhaust gas containing a wax vapor discharged from a sintering furnace for sintering a powder molding product and collecting it therefrom.
However, in the conventional trap apparatus as described in the aforementioned Japanese Patent Laid-open Application No. 2001-214242, if captured materials are attached to the cooling fins as the trap process progresses, cooling efficiency of the exhaust gas gets declined since the exhaust gas exchanges heat with the cooling fins through a captured material layer. For the same reason, collection efficiency becomes lowered due to aging, so that the impurity gases could not be completely removed and the increasing in the frequency of maintenance and repair becomes problematic. For preventing deterioration of collection efficiency due to aging, it may be considered that the cooling fins are set in multi-levels. However, it is impractical since the apparatus becomes significantly enlarged in this case. Moreover, in case of removing the captured materials from the cooling fins by a cleaning operation during the maintenance and repair, it becomes difficult to perform the cleaning operation since the cooling fins are formed in multi-levels, and thus a whole structure is complicated.
Further, in the trap apparatus as described in Japanese Patent Laid-open Application No. S62-4405, in case where the captured materials are of a semisolid mass having a viscosity, the small holes of the circular trap plate are getting clogged with the captured materials, so that the increasing in the frequency of maintenance and repair becomes problematic.
Still further, in the trap apparatus as described in Japanese Patent Laid-open Application No. S62-4405, cooling efficiency is not very high since the exhaust gas is adiabatically expanded through the small holes. Thus, the impurity gas in the exhaust gas is not fully collected, thereby lowering collection efficiency.